'No to alternative vote' camp gains record poll lead

Guardian/ICM poll puts no vote on 44% and yes vote on 33%, but turnout may yet swing outcome of electoral reform

Time is running out for the pro-AV campaign to win people round before the 5 May referendum. On Monday, comedian Eddie Izzard joined Labour leader Ed Miliband and business secretary Vince Cable in London to promote the alternative voting system. Photograph: Stefan Rousseau/PA

Opinion polls disagree on the precise outcome of the alternative vote referendum – but the movement has mostly been in one direction. Today's Guardian/ICM poll gives the no camp a record lead, but since the start of the year almost all polls have shown a shift away from earlier support for a different electoral system.

Many voters remain unengaged in the campaign but those that are seem less convinced than they were of the need for change. Crucially, groups that are more likely to vote, such as pensioners, also seem more likely to vote no than average.

The actual result may come down to a difference in turnout, which is taken into account in today's Guardian/ICM poll and many carried out by other companies online.

Estimating turnout before an election is difficult, since more voters claim they will go to the polls than actually do so. The difference between people who tell pollsters they are likely to vote and those who actually do on 5 May could yet swing the outcome.

Devolved elections in Scotland, Wales and Northern Ireland and local elections in much of England will also have an effect. The Guardian/ICM results assume that people who say they will vote in one of the other contests will also use their right to vote on alternative vote (AV).

Without adjusting for turnout, and including people who have not made up their minds, the latest Guardian/ICM poll puts the yes vote on 33%, nos on 44% and don't knows on 23%. In the same poll in February the figures were 37%, 37% and 27%. After adjusting for turnout and including only those people who have an opinion, 58% back first past the post and 42% want the alternative vote. In February the figures were the other way around: 49% for first past the post and 51% for AV.

Across the United Kingdom, including Northern Ireland, 50% say they are certain to vote on 5 May, with another 16% saying they are likely to do so. Scottish voters are the most likely to vote, followed by Wales, England and Northern Ireland.

In the parts of England where there are no local elections, only the referendum, 44% say they are certain to vote — though the referendum turnout in places such as London is expected to be much lower.

Areas in England with no council elections – led by London – are the only ones where support for AV is now ahead. This implies the yes campaign has to get its vote up in the capital to succeed.

Time is running out for the yes campaign to win people round. In some wards up to half of voters are registered for postal votes, and many will be returning them this week. Easter and the run-up to the royal wedding will also make it harder for campaigners to get their case across.

The Guardian/ICM poll stands out because it was carried out using a traditional random sample of voters contacted by telephone – the first poll of this kind for two months. But the trend reflects other more recent online polls.

At the start of March, YouGov reported a three-point lead among all voters for a switch to AV. Its most recent poll, before adjustment for turnout, put first past the post ahead by three points. Other recent online polls from ComRes and YouGov have shown slightly larger "no" leads.

Most polls, including the Guardian/ICM series, have asked people to respond to the question that will be asked on the referendum ballot paper.

There is little significant difference between men and women in attitudes to AV, but a clear split between supporters of different parties. Labour voters are split; Lib Dems in favour and Conservatives against.

Researchers interviewed a random sample of 1,033 adults across the United Kingdom aged 18+ by telephone on 15-17 April 2011. Interviews were conducted across the country and the results have been weighted to the profile of all adults. Voting intention based on British sample of 1,003 people.